Adapt context bound evidence to implicits in last parameter list

If we have a function like

```scala
def foo[X: CB](...)(implicit x: T)
```
always map context bounds to implicit parameters, irrespective of version.

Likewise, if we have a function
``scala
def foo[X: CB](...)(using x: T)
```
always map context bounds to "using" parameters, irrespective of version.
This avoids mixing implicit and using in one parameter list.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -300,7 +300,12 @@ object desugar {
       // implicit resolution in Scala 3.
 
     val paramssNoContextBounds =
-      val iflag = if Feature.sourceVersion.isAtLeast(`3.6`) then Given else Implicit
+      val iflag = paramss.lastOption.flatMap(_.headOption) match
+        case Some(param) if param.mods.isOneOf(GivenOrImplicit) =>
+          param.mods.flags & GivenOrImplicit
+        case _ =>
+          if Feature.sourceVersion.isAtLeast(`3.6`) then Given
+          else Implicit
       val flags = if isPrimaryConstructor then iflag | LocalParamAccessor else iflag | Param
       mapParamss(paramss) {
         tparam => desugarContextBounds(tparam, evidenceParamBuf, flags, freshName, paramss)
@@ -472,7 +477,14 @@ object desugar {
       case ValDefs(mparams) :: _ if mparams.exists(referencesBoundName) =>
         params :: mparamss
       case ValDefs(mparams @ (mparam :: _)) :: Nil if mparam.mods.isOneOf(GivenOrImplicit) =>
-        (params ++ mparams) :: Nil
+        val normParams =
+          if params.head.mods.flags.is(Given) != mparam.mods.flags.is(Given) then
+            params.map: param =>
+              val normFlags = param.mods.flags &~ GivenOrImplicit | (mparam.mods.flags & (GivenOrImplicit))
+              param.withMods(param.mods.withFlags(normFlags))
+                .showing(i"ADAPTED PARAM $result ${result.mods.flags} for ${meth.name}")
+          else params
+        (normParams ++ mparams) :: Nil
       case mparams :: mparamss1 =>
         mparams :: recur(mparamss1)
       case Nil =>